The diagnostics and monitoring of structures for damage, such as that carried out in the structural health monitoring (SHM) field, are often accomplished by employing arrays of transducers. The transducers are often used as both actuators and sensors, and typically comprise piezoelectric transducers, e.g., lead-zirconate-titanate (PZT) transducers, which are bonded to the structure to excite elastic waves at ultrasonic frequencies that propagate along paths on the surface of the structure or through the structural medium. When damage occurs on or in the structure between the transducers, the associated actuator-sensor propagation paths are affected by the damage, resulting in changes to the signals received at the sensors indicative of the location, size and type of the damage. Knowing which actuator-sensor paths are affected is useful in determining the presence of damage and the approximate location of damage. However, there may not be direct information about the size of the damage. There is a need, therefore, for a method for determining the size of the damage, as well as methods for placing probabilistic bounds on the size of the damage detected.